The present invention relates to a process for preparing 4-acetoxy-3-hydroxyethylazetidin-2-one which has a hydroxyethyl group, wherein the hydroxyl group is protected, at the C3-position and has an acetoxyl group at the C4-position. It is known that 4-acetoxy-3-hydroxyethylazetidin-2-one derivatives are useful intermediates for preparing carbapenem .beta.-lactam antibiotics such as thienamycin and penem .beta.-lactam antibiotics (cf., for example, Tetrahedron Letters by Reider et al., vol. 23, page 2293, 1982 and Chem. Pharm. Bull. by Yoshida et al., vol. 29, page 2899, 1981).
There hitherto have been known processes for synthesizing 4-acetoxy-3-hydroxyethylazetidin-2-one derivatives, for instance, synthesis from 6-aminopenicillanic acid (cf. Chem. Pharm. Bull. by Yoshida et al., vol. 29, page 2899, 1981), synthesis from threonine (cf. Tetrahedron by Shiozaki et al, vol. 39, page 2399, 1983) and synthesis from aspartic acid (cf. Tetrahedron Letters by Reider et al., vol. 23, page 2293, 1982). However, these processes have a problem that industrially unfavourable heavy metals such as mercury acetate and lead tetraacetate are employed in order to introduce an acetoxyl group at the C4-position of the .beta.-lactam ring.